Method of measuring pressure by means of a pressure gauge having a resonant element

ABSTRACT

The invention relates to a method of measuring pressure in which an evacuated capsule ( 1 ) containing a resonant element ( 5 ) is placed in the fluid whose pressure is to be measured, a vibration characteristic of the element is measured, and the pressure is deduced from said characteristic. A resonant element is used which, during measurement, is to be found in a stress state that is close to buckling. For this purpose, it is possible to use heater means for heating the element and servo-controlled so as to keep the frequency of vibration thereof constant. The resonant element can be made of silicon. The invention is particularly applicable to oil wells.

The present invention relates to a method of measuring pressure by meansof a pressure gauge having a resonant element, in particular for use inoil wells, and more particularly the invention relates to such a methodin which an evacuated capsule containing a resonant element is placed inthe fluid whose pressure is to be measured, a resonance characteristicof the element is measured, and the pressure is deduced from saidcharacteristic.

Document U.S. Pat. No. 4,547,691 discloses sensors of this type, knownas crystal quartz gauges (CQGs) in which the evacuated capsule and thevibrating beam it contains are both made of quartz. Such sensors havethe advantages of being capable of operating in a hostile environment,at very high pressure (up to 1500 kg/cm²), and at high temperatures (upto 200° C.), thereby making them particularly suitable for use in thefield of oil exploration, particularly for continuous monitoring ofdeposits. Finally, they present very good accuracy, of the order of0.01% of full scale.

The quartz beam is set into resonance by the piezoelectric effect andthe frequency of its vibration is measured accurately. The pressure towhich the cell is subjected is deduced therefrom. Since the frequency ofvibration is relatively insensitive to the effects of aging, of drift,of fatigue due to stress relaxation, of thermoelectric effects, or ofinstability of the electronics, such quartz sensors are therefore alsovery stable and of good resolution, unlike membrane or strain-gaugepressure sensors.

Nevertheless, they have the drawback of being high in cost and ofpossessing dimensions that are relatively large.

The present invention seeks to mitigate those drawbacks.

More particularly, the invention seeks to provide a pressure gauge thatis capable of operating under the same conditions as a quartz pressuregauge and of providing a measurement of the same accuracy, but which ismuch easier to industrialize and which is of much lower cost.

Still more particularly, the invention seeks to provide both a method ofmeasuring pressure and a pressure gauge which can be used on a largescale in the oil industry, particularly for continuous monitoring ofdeposits, but which could also be used while drilling.

To this end, the invention firstly provides a method of measuringpressure by as pressure gauge having a resonant element, in particularfor use in oil wells, in which an evacuated capsule containing aresonant element is placed in the fluid whose pressure is to bemeasured, a resonance characteristic of the element is measured, and thepressure is deduced from said characteristic, the method beingcharacterized by the fact that a resonant element is used which is to befound, during measurement, in a stress state that is close to buckling.

It will be observed that the invention relates to sensors having aresonant element, i.e. sensors of the same type as the above-mentionedquartz sensors, and not membrane or strain-gauge sensors. Advantage isthus taken of the fact that the magnitude being measured is associatedwith a frequency of vibration and not with a deformation.

In addition, the sensitivity of the method of the invention is increasedby the fact that the resonant element is in a stress state that is closeto buckling. Under such circumstances, the stiffness of the beam tendstowards zero. A very small variation in the compression exerted thereontherefore gives rise to a considerable variation in its frequency ofvibration. This provides mechanical amplification of the sensitivity ofthe sensor.

Advantageously, the resonant element is made of silicon.

This makes it possible to benefit from the excellent mechanicalproperties of silicon, particularly when it is monocrystalline. Inaddition, technology associated with using monocrystalline silicon isthoroughly mastered and well adapted to mass production.

It will be observed that, other things being equal, the sensitivity of aresonant element sensor made of silicon would normally be much less thanthat of an equivalent quartz sensor. The method of the invention makesit possible to remedy that drawback because measurement is performedwhile the resonant element is in a stress state that is close tobuckling.

Two implementations of the method can be envisaged.

Firstly, it is possible merely to excite the element to resonance andmeasure its frequency of vibration.

The sensor is then operating in an open loop mode. The stress state ofthe resonant element, and thus its frequency of vibration, depend on thepressure to be measured.

In the other implementation, the stress state of the resonant element isadjusted to the limiting condition for buckling within the range ofpressures to be measured.

Its frequency thus remains constant, and pressure is measured bymeasuring the stress applied to the resonant element. The sensor is thenoperating in a servo-controlled mode.

In a particular implementation, it is the temperature of the resonantelement that is adjusted, e.g. using the Joule effect, so as to keep itin a stress state close to buckling.

Under such circumstances, a thermal expansion stress is thus added tothe mechanical stress exerted on the element by the pressure that is tobe measured, with this being done in such a manner as to reach thedesired proximity of the buckling state. The resonant element alwaysvibrates at the same frequency and the electrical current used foradjusting its temperature is representative of the pressure to bemeasured.

The invention also provides, a pressure gauge having a resonant element,suitable for performing measurements in oil wells, the gauge comprisinga resonant element of crystalline material placed in an evacuatedcapsule organized to be subjected to a pressure that is to be measured,means for setting said element into resonance, and means for deducingthe pressure from a vibration characteristic of the element, the gaugebeing characterized by the fact that stress control means are providedto ensure that during measurement the resonant element is to be found ina stress state close to buckling, thereby amplifying the sensitivity ofthe sensor.

More particularly, the resonant element can be made out of silicon.

In a first embodiment, said vibration characteristic of the resonantelement is its frequency of vibration.

In another embodiment, said vibration characteristic of the resonantelement is an electrical magnitude to which said means need to besubjected in order to ensure that the element keeps a frequency ofvibration that is constant in spite of pressure variations.

It will also be observed that in prior art quartz sensors, vibration isexcited by piezoelectric means. Other means need to be envisaged for asensor having a resonant element made of silicon, since silicon is notpiezoelectric.

In the invention, the excitation means can be capacitive means oroptical means.

In a particular embodiment, said means for operating during measurementto cause the resonant element to be found in a stress state close to itslimit condition for buckling can be means for heating the element, inparticular by the Joule effect.

A particular embodiment of the invention is described below by way ofnon-limiting example with reference to the accompanying diagrammaticdrawing, in which:

FIG. 1 shows a pressure gauge of the invention with its measurement cellin an exploded perspective view; and

FIG. 2 is a diagram illustrating the general operation of the device.

The measurement cell 1 of the FIG. 1 pressure gauge is made in threeparts, namely: a hollow package 2 and two covers 3 a and 3 b.

The package 2 forms a rigid frame 4. A resonant element, in this case avibrating beam 5, forms, a bridge between two opposite sides of theframe 4. The cross-section of the beam 5 is small compared with that ofthe sides of the frame 4 such that the beam can be considered as beingin compression between two fixed ends.

Each cover 3 a and 3 b is constituted simply by means of a plane platebonded to the package by any appropriate means so as to enclose the beam5 in an evacuated enclosure.

The package 2 including the vibrating beam 5, and the covers 3 a and 3 bare all made of silicon. The technique used can be the technique ofanisotropic chemical machining, e.g. using potassium hydroxide, as isknown in microelectronics.

The faces of the covers 3 a and 3 b that come into contact with thepackage 2 are oxidized, e.g. thermally, so that the resulting layer ofsilica 6 provides electrical insulation.

The covers 3 a and 3 b can be mounted on the package 2 by direct bondingor by anode soldering a thin layer of powdered glass.

The electronics essentially comprises an oscillator circuit 7 and aservo-control circuit 8 operating on the limiting condition for beambuckling.

The oscillator circuit 7 operates in conventional manner to providecapacitive excitation of the beam 5 and to measure its resonancefrequency f. The circuit 7 is shown here as being connected to the cover3 a and to the frame 4 of the package 2.

In FIG. 2, it can be seen how interactions take place between theservo-control circuit 8, the beam 5, and the oscillator circuit 7. Thepressure P of the external medium is added in the measurement cell 1 tothe pressure P_(th) due to the resistive heating of the beam 5 so as togive the pressure P_(f) that compresses the beam 5 longitudinally.

The beam 5 is excited into vibration capacitively by the oscillatorcircuit 7 and it thus vibrates at its resonance frequency f. Thisfrequency is compared in a comparator 9 with a reference f₀ that issuitably selected to correspond to the beam being in a state where it isin a limiting condition for buckling.

The frequency difference is used to control a current generator 10. Thecurrent delivered by the generator 10 is injected into the beam 5 togenerate the pressure P_(th) which ensures the limiting condition inbuckling.

The frequency difference supplied by the comparator 9 (or the currentsupplied by the generator 10) is also representative of the pressure P.This difference (or current) is therefore transmitted to a circuit (notshown) which gives the pressure directly.

Maximum sensitivity is thus made available since the resonance frequencyof the beam 5 varies very quickly as a function of the appliedcompression, and thus as a function of the applied pressure P, when thebeam is close to buckling.

By way of example, the vibration modes of a silicon beam that is 0.25 mmlong, 0.05 mm wide, and 4.8 μm thick have been computed. Under theconditions used for the computation, the buckling pressure was 1180bars.

In the absence of any applied pressure, the natural frequency of thefirst mode of vibration was 657 kHz. Its sensitivity to pressure in therange 0 to 800 bars in said first mode was about 360 Hz/bar. Thissensitivity increased significantly for higher pressures and exceeded2000 Hz/bar in the range 1100 to 1180 bars.

The above description relates only to the embodiment in which frequencyexcitation/detection is of the capacitive type. In a variant it ispossible to provide for excitation/detection to be of the optical type.Under such circumstances, vibration is generated by heating the beam 5using light pulses delivered by an optical fiber.

The optical embodiment makes it possible to take advantage of the factthat optical fiber is electrically insulating, of small diameter,flexible, and insensitive to electromagnet disturbances.

What is claimed is:
 1. A method of measuring pressure of a fluid in anoil well with a pressure gauge including an evacuated capsule having aresonant element contained therein, the method comprising: (i) placingthe evacuated capsule containing the resonant element in the fluid whosepressure is to be measured; (ii) measuring a vibration frequency f ofthe resonant element while it is in a stress state that is close tobuckling; (iii) comparing this vibration frequency f with a vibrationfrequency reference f₀ that is suitably selected to correspond to theresonant element being in a state where it is a limiting condition forbuckling; and (iv) deducing the pressure of the fluid in the oil wellfrom the comparison between vibration frequency f and the referencevibration frequency f₀.
 2. A method as claimed in claim 1, wherein theresonant element is made of silicon.
 3. A method as claimed in claim 1,further comprising applying an electric signal to the resonant elementin order to maintain a constant frequency of vibration irrespective ofvariation in pressure of the fluid, said constant frequency of vibrationensuring the limiting condition in buckling for the resonant element. 4.A method as claimed in claim 3, wherein the magnitude of the electricsignal that must be applied to the resonant element is representative ofthe pressure of the fluid in the oil well.
 5. A method as claimed inclaim 1, further comprising adjusting the stress state of the resonantelement onto its limiting condition for buckling in the range ofpressures to be measured.
 6. A method as claimed in claim 5, comprisingadjusting the temperature of the resonant element in order to maintain astress state close to buckling.
 7. A method as claimed in claim 5,wherein the step of adjusting the stress state of the resonant elementcomprises adjusting the temperature of the resonant element using theJoule effect.
 8. A method of measuring pressure of a fluid in an oilwell with a pressure gauge including an evacuated capsule having aresonant element contained therein, the method comprising: (i) placingthe evacuated capsule containing the resonant element in the fluid whosepressure is to be measured; (ii) compressing the resonant element with apressure P_(f), said pressure being the result of the addition of thepressure P due to the fluid in the oil well and the pressure P_(th) dueto the resistive heating of said resonant element; (iii) measuring theresonance frequency f of the resonant element, said resonance frequencycorresponding to the pressure P_(f); (iv) comparing the resonancefrequency f with a reference resonance frequency f₀ that is suitablyselected to correspond to the resonant element being in a state where itis a limiting condition for buckling; (v) using the frequency differenceto control the magnitude of the current that must be applied to theresonant element in order to generate the pressure P_(th) that ensuresthe limiting condition in buckling; and (vi) deducing the pressure P ofthe fluid in the oil well from the comparison between vibrationfrequency f and the reference vibration frequency f₀.
 9. A fluidpressure gauge for performing measurements in oil wells, comprising: (i)an evacuated capsule which is immersed in a fluid to be measured; (ii) aresonant element of crystalline material located in the evacuatedcapsule and subjected to a pressure that is to be measured; (iii) aresonator for setting the element into resonance; (iv) stress controlmeans that maintain the resonant element in a stress state close tobuckling during measurement; and (v) a comparator that compares theresonance frequency of the resonant element with a reference resonancefrequency that is suitably selected to correspond to said resonatorelement being in a state where it is a limiting condition for buckling.10. A pressure gauge as claimed in claim 9, further comprising means fordeducing fluid pressure from a vibration characteristic of the element.11. A pressure gauge as claimed in claim 9, wherein the resonant elementis made of silicon.
 12. A pressure gauge as claimed in claim 9, whereinsaid stress control means comprise a generator that applies anelectrical signal, to the resonant element to maintain a constantfrequency of vibration irrespective of variations in fluid pressure,said constant frequency of vibration ensuring the limiting condition inbuckling for the resonant element.
 13. A pressure gauge as claimed inclaim 9, wherein the resonator comprises capacitive means.
 14. Apressure gauge as claimed in claim 9, wherein the resonator comprisesoptical means.
 15. A pressure gauge as claimed in claim 9, wherein thestress control means comprises means for heating the resonant element.